FRESH-WATER ALG. 209 
liar species grows in broad velvet-like patches of a 
dark-green colour. Their vitality is so great that 
they are capable of enduring the extremes of heat 
and cold, for they have been found on fragments 
of ice in Melville Island, where the temperature 
is considerably below zero; and they have been 
found growing like vegetable salamanders in 
thermal springs in different parts of the globe, 
where the heat is sometimes so great that the in- 
habitants of the surrounding districts dress their 
food over them, and use them for other economic 
purposes instead of fire. Sir Humphry Davy in 
his Consolations in Travel mentions that the float- 
ing islands of Oscillatoriz, which are constantly 
found in Lake Solfatara in Italy, exhibit a striking 
example of the luxuriance of cryptogamic vege- 
tation in an atmosphere impregnated with carbonic 
gas. They are supplied by this gas, which is 
constantly escaping from the bottom of the lake, 
with a violence that causes the water to boil. 
A magnificent species forms thick woolly fleeces 
of a deep red colour, in the ponds and collections 
of water in the central and western districts of 
India, occurring in great profusion in the hot, 
sweltering valleys of the great Runjeet, ascending 
into Nepaul and the lower slopes of the Hima- 
layas. The most singular member of this curious 
group, however, is the 77v¢chodesmium erythreum 
oO 
